Radius of Cation is Less and Anion is More Than Its Parent Atom, Size Variation in Isoelectronic Series
The radius of a cation is always smaller than its parent atom, while the radius of an anion is larger due to changes in electron number and effective nuclear charge. When an atom loses electrons to form a cation, reduced electron-electron repulsion and increased nuclear attraction pull the remaining electrons closer, decreasing size. In contrast, when an atom gains electrons to form an anion, increased repulsion between electrons causes expansion of the electron cloud. In an isoelectronic series, where species have the same number of electrons, size variation depends on nuclear charge—the greater the nuclear charge, the smaller the radius.